Decline of the invasive plant Asparagus asparagoides within the first seven years after release of biological control agents in Australia.
There is ample anecdotal evidence and expert opinion on the adverse impact of imported biological control agents on populations of invasive plants, but still a paucity of quantitative data. Asparagus asparagoides (bridal creeper), a perennial climber with an extensive below-ground network of rhizomes and tubers, was considered one of the most invasive plants of natural ecosystems in southern Australia in the 1990s. A long-term experiment was conducted at 15 sites invaded by A. asparagoides across Australia to determine whether its growth and reproduction declined following the release of two biological control agents: a leafhopper (undescribed Erythroneurini formerly referred to as Zygina sp.) and rust fungus (Puccinia myrsiphylli). Data on A. asparagoides were collected annually at each site for up to 3 years before the release of one or both agents in 2000 or 2001, and up to 7 years after release to capture spatial and temporal variability. Our results showed a steady decrease in A. asparagoides seedling and shoot density, and total above-ground biomass in quadrats across all sites in the years following the release of the leafhopper and/or rust fungus. The number of fruits produced in quadrats greatly varied between sites, although for most sites, fruit numbers before the release of agents were higher than after release. Changes in A. asparagoides climbing onto standardized trellises, however, were not consistent across sites during the same period, with major reductions in plant measurements recorded at some sites but not others. Differences in climatic conditions between sites during A. asparagoides growing season did not explain these variable results. Incidence of leafhopper damage on A. asparagoides cladodes in quadrats and trellises was generally lower than that of rust infection.